Vitamin A suppresses vitamin D's effect

The British Medical Journal has published a remarkable paper confirming that low vitamin D levels obtained in the past are a risk factor for developing colon cancer in the future. But the contained an even more significant finding -- as this site has reported before, vitamin A, even in relatively low amounts, can thwart vitamin D's association with reduced rates of colon cancer. This is the largest study to date showing vitamin A blocks vitamin D's effect.

Hidden on page eight of the paper was one sentence and a small table, showing that the benefits of vitamin D are almost entirely negated in those with the highest vitamin A (retinol) intake. And the retinol intake did not have to be that high -- only about 3,000 IU/day. Young autistic children often take 3,500 IU of retinol a day in their powdered multivitamins, which doesn't count any additional vitamin A given in high single doses.

The finding explains some of the anomalies in other papers on vitamin D and cancer -- similar studies sometimes have widely different results. This may be because the effect of vitamin A was not taken into account.  In some countries, cod liver oil, which contains vitamin A, is commonly used as a vitamin D supplement, and in others it is used more rarely, causing differences in the results.